I just checked my car...and when I have my phone paired and using BT connection...-1 appears if I go back to the nav map. I only have 1 phone paired which is visibly listed under external devices.
Then, if I switch to USB (I have a thumb drive inserted in the center armrest compartment)...the symbol changes to the USB icon & whatever the track number playing is...thats the nbr that appears. Same goes for a CD/DVD inserted...the symbol changes to a disc...and the track nbr playing shows next to it. Then if I select music collection (song tracks uploaded from a CD to the hard drive)...a different symbol appears & the track nbr appears to the right.
If the number keeps changing...are you listening to different sources? Or does the BT source you're listening to has multiple tracks? I was listening to a mixed artist Pandora station. If it is a single artist's album you're listening to...perhaps it is album track nbrs.